Field of the Invention
The invention relates to a process of, and apparatus for, treating sewage while it is being held in or is flowing through a rising or pumping sewer.
Description of the Prior Art
Problems can arise in conveying sewage through rising or pumping sewers as a result of the action of facultative bacteria present in the sewage. These bacteria normally respire oxygen dissolved in the sewage, but if this supply of oxygen is exhausted, they obtain their oxygen for respiration by reducing compounds containing oxygen, particularly sulphates.
This bacterial reduction of sulphates yields hydrogen sulphide, a malodorous, poisonous gas. Sometimes lethal concentrations of hydrogen sulphide are created at locations that are accessible to maintenance engineers, and there is frequently escape of hydrogen sulphide from the sewer to above ground. In particular, hydrogen sulphide can be formed in rising main sewers in view of the exclusion of air from between the pumphouse and the discharge end of the sewer. In consequence the presence of hydrogen sulphide is often detected just beyond the outlet of a rising sewer.
Formation of hydrogen sulphide is particularly marked under hot climatic conditions, since these conditions promote bacterial activity.
Various methods have been tried in order to prevent the bacterial activity which forms hydrogen sulphide. For example, bacterial activity can be inhibited by the addition of gaseous chlorine, but this can result in the sterilisation of the sewage. Moreover, chlorine itself is both highly corrosive and toxic.
It has been proposed to freshen sewage by adding air. With air, however, once a concentration of dissolved oxygen of 10 ppm has been attained, no more oxygen will dissolve. Furthermore, the sewage is usually saturated with nitrogen before the air is introduced. Accordingly, none of the nitrogen contained in the air is dissolved. This undissolved nitrogen can cause gas locks and other hydraulic problems in the sewer.
One example of a proposal to introduce oxygen into a gravity sewer is contained in U.S. Pat. No. 3,525,685 (Edwards).
Air at atmospheric pressure or other oxygen-containing gas is bubbled into sewage flowing through a gravity sewer. The air or other oxygen-containing gas is intended to escape from the liquid sewage surface and force the lighter sewage gases out of the sewage system, thus allowing a constantly changing liquid surface to further promote oxygen absorption by the sewage. Although it is stated in U.S. Pat. No. 3,525,685 that the invention described therein may be applied to a force mains this would in fact result in most of the gas remaining undissolved. This will give rise to `plugs` of gas which will expand as the pressure along the main reduces, thereby causing violent geysering and quantities of sewage being ejected, typically 30 or 40 feet in an upwards direction, at the discharge of the main. Moreover, conventional pumped sewage mains are built such that they follow the topography of the land. This generally requires the installation of several siphons and inverted siphons along the length of the main. Undissolved gas will collect in the siphons and thereby increase the pressure required to pump sewage through the main and severely reduce the pumping flow rate. There is thus a considerable increase in the power requirements of the pumps.
In U.S. Pat. No. 3,335,082 it is proposed to pass air along the ullage space of a pipe forming part of a gravity sewer. However, this technique is inapplicable to a rising or pumping sewer as such sewers do not have pipes with ullage spaces.
Proposals have been made to introduce pure oxygen into sewage being treated in the secondary stage of an activated sludge plant. Such a process is described in U.S. Pat. No. 3,607,735.